Oxidation under Air of Tavorite LiVPO₄F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties

Abstract

Tavorite-type compositions offer a very rich crystal chemistry, among which LiV III PO 4 F has the highest theoretical energy density (i.e. 655 Wh/kg). In this article, an in-depth study of vanadyl-type defects generated by temperature-controlled oxidation of LiV III PO 4 F under air is proposed and their influence on the electrochemical properties are demonstrated. A combination of high resolution synchrotron diffraction, infrared spectroscopy and magic angle spinning nuclear magnetic resonance was used to fully characterize the materials thus generated, from their average long range structure to their local structure with the presence of defects. The increase of the annealing temperature tends to substitute oxygen for fluorine with the formation of a series of LiVPO 4 F 1-x O x compositions. The miscibility domains appear to be narrow at the two ends of the solid solution (i.e. in the composition ranges LiVPO 4 F [1-0.9] O [0-0.1] and LiVPO 4 F [0-0.1] O [1-0.9]). The presence of vanadyl-type defects obtained as localized or more extended ones, depending on the annealing conditions, affects drastically the electrochemical properties of these Tavorite LiVPO 4 F-type materials.

LiVPO4F1–yOy Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance

“…The profiles of all these patterns are governed by a strong anisotropic strain broadening and must be refined using the corresponding model provided in the Fullprof software 13 . As already observed for that kind of materials 17,19 , microstrains are mainly observed along the propagation direction of the chains of octahedra (i.e.…”

“…with N the number of bond distances considered, di the distance between the atom i and the central atom of the polyhedron and ‹d› the average of the distances considered. Those appearing as filled and colored symbols are at the center of this paper, whereas those appearing as black and empty symbols are already reported in literature, either by other authors 6,18 or by us 17 . The red-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4F-type (Z = 2, with centrosymmetric vanadium octahedra) and the blue-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4O-type (Z = 4, with distorted vanadium octahedra).…”

“…Nevertheless, a tiny peak is observed at 874 cm -1 , shifted versus the vanadyl stretching mode observed for LiVPO4O, but matching with the presence of local vanadyl-type defects in LiVPO4F. 17 As the average oxidation state of vanadium is increased, a slight shift towards higher wavenumbers and an increasing amplitude of the vanadyl-type signal are observed: Note that the intensity of the vanadyl vibration doesn't evolve linearly with the O 2- 25 , around 875 cm -1 for each sample. Weaker ones, at slightly higher wavenumber (~911 cm -1 ), are also observed for mixed valence materials with a broadening of the signal.…”

A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO4F

“…[44][45] For example, in tavorite LiVPO 4+x F 1−x compounds, the average voltage and reversible specific capacity at practical current rates decrease as the fluorine content decreases.…”

“…[44][45] To better understand the role of the fluorine:oxygen ratio in KVPO 4 F, we prepared an oxygen-substituted composition, [31,33] but slightly smaller than the values obtained for our stoichiometric KVPO 4 F sample, as shown in Figure 1a and Supplementary Elemental analysis using inductively coupled plasma for K, V, P, and the ISE method for between ~800 ppm and ~1400 ppm, while the itself is comprised between ~1600 and ~3200 ppm. [48] Hence, it is not unlikely that corrections of a similar order of magnitude may need to be applied to our calculation results on KVPO 4 F, but this should not significantly affect the shift differences in Supplementary Table 4.…”